On a failover cluster that uses Cluster Shared Volumes, multiple clustered virtual machines that are distributed across multiple cluster nodes can all access their Virtual Hard Disk (VHD) files at the same time, even if the VHD files are on a single disk (LUN) in the storage. This means that the clustered virtual machines can fail over independently of one another, even if they use only a single LUN.

In contrast, in a failover cluster on which Cluster Shared Volumes is not enabled, a single disk (LUN) can only be accessed by a single node at a time. This means that clustered virtual machines can only fail over independently if each virtual machine has its own LUN, which makes the management of LUNs and clustered virtual machines more difficult.

This topic contains the following sections:

Benefits of using Cluster Shared Volumes in a failover cluster

Cluster Shared Volumes provides the following benefits in a failover cluster:

  • The configuration of clustered virtual machines is much simpler than before.

  • You can reduce the number of LUNs (disks) required for your virtual machines, instead of having to manage one LUN per virtual machine, which was previously the recommended configuration (because the LUN was the unit of failover). Many virtual machines can use a single LUN and can fail over without causing the other virtual machines on the same LUN to also fail over.

  • You can make better use of disk space, because you do not need to place each Virtual Hard Disk (VHD) file on a separate disk with extra free space set aside just for that VHD file. Instead, the free space on a Cluster Shared Volume can be used by any VHD file on that volume.

  • You can more easily track the paths to VHD files and other files used by virtual machines. You can specify the path names, instead of identifying disks by drive letters (limited to the number of letters in the alphabet) or identifiers called GUIDs (which are hard to use and remember). With Cluster Shared Volumes, the path appears to be on the system drive of the node, under the \ClusterStorage folder. However, this path is the same when viewed from any node in the cluster.

  • If you use a few Cluster Shared Volumes to create a configuration that supports many clustered virtual machines, you can perform validation more quickly than you could with a configuration that uses many LUNs to support many clustered virtual machines. With fewer LUNs, validation runs more quickly. (You perform validation by running the Validate a Configuration Wizard in the snap-in for failover clusters.)

  • There are no special hardware requirements beyond what is already required for storage in a failover cluster (although Cluster Shared Volumes require NTFS).

  • Resiliency is increased, because the cluster can respond correctly even if connectivity between one node and the SAN is interrupted, or part of a network is down. The cluster will re-route the Cluster Shared Volumes communication through an intact part of the SAN or network.

Restrictions on using Cluster Shared Volumes in a failover cluster

The following restrictions apply when using Cluster Shared Volumes in a failover cluster:

  • The Cluster Shared Volumes feature is only supported for use with Hyper-V (a server role in Windows Server 2008 R2) and other technologies specified by Microsoft. For information about the roles and features that are supported for use with Cluster Shared Volumes, see http://go.microsoft.com/fwlink/?LinkId=137158.

  • No files should be created or copied to a Cluster Shared Volume by an administrator, user, or application unless the files will be used by the Hyper-V role or other technologies specified by Microsoft. Failure to adhere to this instruction could result in data corruption or data loss on shared volumes. This instruction also applies to files that are created or copied to the \ClusterStorage folder, or subfolders of it, on the nodes.

  • For Hyper-V to function properly, the operating system (%SystemDrive%) of each server in your cluster must be set so that it boots from the same drive letter as all other servers in the cluster. In other words, if one server boots from drive letter C, all servers in the cluster should boot from drive letter C.

  • The NTFS file system is required for all volumes enabled as Cluster Shared Volumes.